Coherent and non-coherent averaging functions, two well known functions, are performed on a received signal in a receiver portion of a communication system for many different applications. In particular in a code division multiple access communication system, for example, one or more of the averaging functions are performed on a received signal to mitigate some undesired effects of the received signal distortions due to fading and additive noise plus interference. The results of averaging are used to generate a power-delay profile of the channel through which the received signal has propagated. The power-delay profile is then typically used to estimate time delay and amplitude of the received signal to perform demodulation of the received signal in a Rake receiver in the code division multiple access communication system. Moreover, time delay and amplitude of the received signal are used to determine location of a remote communicating unit. For example, time delay and amplitude of the received signal are used in a method disclosed in the issued U.S. Pat. No. 5,786,791, to Bruckert, assigned to Motorola Inc., assignee of the present invention, and incorporated herein by reference, for determining an angle of arrival of a signal transmitted by a remote communicating unit in a communication system for determining location of the remote communicating unit.
In general, the averaging functions are performed over a limited interval to determine a power-delay profile of the received signal. The phase information is lost in non-coherent averaging as is well known to one ordinary skilled in the art. In contrast, in coherent averaging, the phase information is always preserved. Moreover, advantages of coherent averaging in many different applications in communication systems are well known. As the result of preserving the phase information, more accurate power-delay profile of the received signal than non-coherent averaging at the same signal to noise ratio is produced.
Doppler frequency effects the phase of the received signal among other effects. The Doppler frequency produces phase rotation of the received signal at a proportional rate. Consequently, two samples, namely samples in complex notation, may have 180 degrees phase rotation from each other due to the phase rotation caused by the Doppler frequency. When the complex samples of the received signal have substantial phase differences, the advantage of coherent averaging of the complex samples diminishes which then produces a less accurate power delay-profile of the received signal. If the averaging interval is chosen to be large, as a means to reduce the effect of the Doppler phase rotation, the result of the coherent averaging approaches zero assuming the noise was additive. On the other hand, if the coherent averaging interval is chosen to be small, as a means to reduce the effect of the Doppler phase rotation, the noise variance remains to be large and causes error in the power delay profile of the received signal.
Referring to FIG. 1, a block diagram of a power-delay profile generator 100 is shown that may be incorporated in a receiver portion of a code division multiple access (CDMA) communication system. Power-delay profile generator 100 may be incorporated in a searcher element, as commonly referred to by one ordinary skilled in the art, of the receiver portion. Power-delay profile generator 100 receives a code modulated signal 104 at an input of a despreader 102. Code modulated signal 104 has propagated through a channel before arriving at power-delay profile generator 100. Despreader 102 despreads code modulated signal 104 using a locally generated replica of the spreading code to produce complex samples 103 of code modulated signal 104. The operation of despreader 104 is well known by one ordinary skilled in the art. The duration of despreading function in despreader 102 may be equal to many times the chip time of the modulating code, e.g. 256 times the chip duration. One chip time, Tc, in a code division multiple access communication system, is equal to duration of one clock time of the code modulating sequence that is used to code modulate received signal 104. For example, in a well known code division multiple access communication system operating according to commonly known IS-95 standard, Tc is equal to 1/1.2288 Mcps which is equal to 0.813 micro seconds. Despreader 102 uses pre-assigned code information, and possibly with the use of a sliding correlator, to generate complex samples 103. Complex samples 103 are input to a coherent averaging block 105. Coherent averaging block 105, after receiving a number (N) of complex samples 103, performs a coherent averaging function over the "N" complex samples 103 to produce a coherently averaged complex sample 106. The coherent averaging may be performed according to the following: ##EQU1## where "S(n)" is the received complex sample for each complex sample from n=1 to N. One ordinary skilled in the art may appreciate that coherent averaging may be performed according to the following: ##EQU2## where "W(n)" is a weighting coefficient for received complex sample S(n) for each complex sample from n=1 to N.
The magnitude of coherently averaged complex sample 106 may be squared in a block 107 to produce a coherently averaged real sample 108. The operation of block 107 may be limited to taking the magnitude of the averaged complex sample 106 to produce averaged real sample 108 as one ordinary skilled in the art may appreciate. Averaged real sample 108 are input to an averaging block 109. Averaging block 109, after receiving a number (M) of averaged real samples 108, performs an averaging function over the number (M) of averaged real samples 108 to produce a power delay sample 110 for generating a power delay profile of the received signal 104. The averaging in block 109 may be according to the following: ##EQU3## where "Y(m)" is averaged real samples 108 for m=1 to M. One ordinary skilled in the art may appreciate that the functions performed in blocks 107 and 109 are in essence in combination equal to a non-coherent averaging function.
According to prior art, an optimum number "N" of complex samples "S(n)" may be determined according to the following: EQU N=1/fD.Ts
where fD is the maximum Doppler frequency experienced by code modulated signal 104 received at power-delay profile generator 100. The parameter Ts is the despreading duration in despreader 102. The number (N) of complex samples, when it is based on the maximum Doppler frequency, is least likely to be an optimum number of complex samples for the coherent averaging function in block 105.
Therefore, there is a need to determine an optimum number of complex samples for performing coherent averaging of code modulated complex signals, and a method for correcting errors in the power-delay profile of the received signal 104 due to Doppler frequency shift and fading.